This invention relates to display systems for displaying continuously updated image information signals arranged in successive frames, employing electro-optic light modulators, and more particularly relates to a scrolling color projection display system in which color bars are repetitively scrolled across a single electro-optic light modulator panel to produce a color display.
Such single panel scrolling color display systems are known. See, for example, commonly assigned U.S. Pat. No. 5,532,763, incorporated herein by reference.
These display systems are particularly suitable for displaying color information in the form of continuously updated image information signals arranged in successive frames representing full color frames, each frame comprised of component color subframes, such as color video information. These systems employ an electro-optic light modulator comprised of a row-and-column matrix array of pixels, for modulating light in accordance with the image information signals during successive frame periods. The signal information is applied to the pixel rows of the array a line at a time during each frame period for each component color.
Such display systems employing a single reflective high-density active-matrix liquid crystal display (AMLCD) on silicon as the electro-optic light modulator are being considered for high resolution color projection systems, due to the fact that the matrix structure of row and column electrodes, switches and storage capacitors can all be integrated on the silicon substrate below the reflective pixel electrodes, thus enabling the high pixel density required for such high resolution displays.
Unfortunately, the location of the reflective pixel electrodes in the LC cell structure leads to an asymmetric electrical behavior during driving, in the form of a DC off-set in the drive voltage, even in the case of AC driving. This DC off-set is perceived as flicker by the viewer of a continuously updated display image, such as a video image
One way to suppress this flicker (George: is this publicly known?) is to adjust the drive voltage to compensate for the DC off-set. However, such a compensation scheme is difficult to implement, since the off-set voltage tends to be non-uniform over the pixel array, and also tends to vary with time.
Another way to suppress this flicker would be to operate the system at a frame rate well above the limit where flicker can be perceived. However, such an operation would reduce the brightness of the display. The reason for this is that guard bars (30, 34 and 38 in FIGS. 2 and 3A) separate the color bars (32, 36 and 40), in order to accommodate the finite switching time of the LC material, and thus preserve color purity.
Operating the system at a higher frame rate) would reduce the size of the color bars (46, 50, 54, 58, 62, 66 in FIG. 3B) in the scrolling direction, since the time period for addressing the pixel array with each new frame of color information would be reduced. However, the size of the guard bars (44, 48, 52, 56, 60, 64) would remain the same, because the switching time of the LC material remains the same. Thus, the proportion of dead time in which the pixel array is not being addressed with color information would be reduced.